Persistent Slabs
- Alpine
- Treeline
- Below treeline
- Likelihood
- Possible
- Size
- 2–3
A variety of weak layers exist in the upper 3' of the snowpack (observation). They appear to be most problematic on middle and upper elevation sunny slopes where they exist as a crust+facet combination. These layers also exist on shadier aspects where an obvious crust may not be present and where we're occasionally finding surface hoar.
The exact location, makeup, and sensitivity of these layers vary and are difficult to pin down. The complexity and spatial variability of this problem make slope scale stability assessment a highly uncertain task. The safest bet is to assume one or more of these layers exist and are capable of producing a large avalanche.
These layers are slowly healing. Slopes steeper than about 35 degrees, and/or those with recent wind loading will take the longest to stabilize. Therefore, avoiding steep, wind-drifted terrain is an effective way to reduce your odds of triggering a large avalanche.
ADDITIONAL DISCUSSION: While triggering slides on faceted snow deep in the snowpack is unlikely, it can't be ruled out in steep, rocky, wind-affected alpine terrain. Slopes like these have variable snow depths, and thinner snowpack areas allow our weight to more easily impact deeply buried weak layers.
(1/1/2023) Debris from a very large avalanche that was remotely triggered above Hyndman Creek in the Pioneer Mountains. This slide failed in a heavily wind loaded area and very likely involved a persistent weak layer. It covered the summer trail with a pile of debris that was 100-150' wide and 4' deep.
